System, method, and apparatus for degassing tool for coal bed methane gas wells

ABSTRACT

A coal bed methane degassing tool is installed at the bottom of a first joint of production casing. As the casing is lowered into the well hole, the tool is fished to the top of the casing as more joints are added. The pressure at the tool may be recorded and logged. Any excessive pressure observed at the tool may be bled off by the tool automatically or manually. As more casing is tripped in, the tool is brought to the top. At the end of the run a log of formation pressure is established. Reservoir engineers can determine reserves in coal bed methane wells drilled and completed before the well is bled off and the tool is removed.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates in general to sensitive formations containing low pressure gas, such as coal bed methane, and, in particular, to an improved system, method, and apparatus for a coal bed methane gas degassing tool.

2. Description of the Related Art

Subterranean deposits of coal contain substantial quantities of entrained methane gas. Limited production and use of methane gas from coal deposits has occurred for many years. Coal seams may extend over large areas of up to several thousand acres, and are fairly shallow in depth, varying from a few inches to several meters, singly or collectively in layers.

After the well is drilled, surface casing is typically installed to a nominal depth. Production casing is then run inside the surface casing to the desired depth of the well. The lowermost joint of the production casing is sealed with a bridge plug as a secondary seal against gas leaks. The primary means of handling the gas during installation of the production casing is an inductor and flare, which receives the gas via the annulus between the surface and production casings. After the production casing is installed, the bridge plug is drilled out to initiate production through the production casing. Although this design is workable, an improved design for sealing production casing during installation would be desirable.

SUMMARY OF THE INVENTION

One embodiment of a system, method, and apparatus for a coal bed methane degassing tool is installed at the bottom of a first joint of production casing. As the casing is lowered into the well hole, the tool is fished to the top of the casing as more joints are added. The pressure at the tool may be recorded and logged. Any excessive pressure observed at the tool may be bled off by the tool. As more casing is tripped in, the tool is brought to the top. Thus, at the end of the run, a log of formation pressure is established. Reservoir engineers can determine reserves in coal bed methane wells drilled and completed with a reverse circulation center discharge (RCCD) process. The well is subsequently bled off and the tool is removed.

The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features and advantages of the invention, as well as others which will become apparent are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only an embodiment of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.

FIG. 1 is a sectional side view of one embodiment of a downhole tool constructed in accordance with the present invention;

FIG. 2 is a schematic view of a well utilizing the downhole tool of FIG. 1; and

FIG. 3 is an enlarged schematic view of an upper portion of the well of FIG. 2 shown utilizing the downhole tool of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, one embodiment of a downhole or degassing tool 11 is shown. The downhole tool 11 comprises a body 13 having an axis 15, a plug shaft 17 including, in one embodiment, a threaded end 19. A fishing shaft 21 is located opposite the threaded end 19, and a bore 23 extends along at least a portion of the axis 15 from the threaded end 19. In the embodiment shown, the plug shaft 17 has a diameter that is less than a diameter of the fishing shaft 21.

A plug 25, such as a cementing plug or slips, is mounted to the plug shaft 17. The plug 25 is a device that frictionally maintains a position of the entire assembly within the production casing. Other mechanical means for securing or locking the tool to the casing also may be used. The plug 25 has an outer diameter 27 for engaging and sealing against the inner wall of production casing, as will be described below. A nut 29 is fastened to the threaded end 19 of the plug shaft 17 for securing the plug 25 to the body 13. A shoulder 35 is formed at an interface between the plug and fishing shafts 17, 21, and the plug 25 is axially restrained between the shoulder 35 and the nut 29.

In one embodiment, the downhole tool 11 also comprises a pressure gage 31 in fluid communication with the bore 23 and extending from the fishing shaft 21. The pressure gage 31 measures pressure in the well downwell from the downhole tool 11. The downhole tool 11 also may comprise a safety relief valve 33 that is also in fluid communication with the bore 23 and extending from the fishing shaft 21. The safety relief valve 33 relieves excessive pressure in the well downwell from the downhole tool 11.

Referring now to FIG. 2, one embodiment of a well system 41 for accessing a subterranean zone from the surface of the well. In the embodiment described below, the subterranean zone is a coal seam. It will be understood that other subterranean formations and/or low pressure, ultra-low pressure, and low porosity subterranean zones can be similarly accessed using the well system of the present invention to remove and/or produce water, hydrocarbons and other fluids in the zone, to treat minerals in the zone prior to mining operations, or to inject or introduce fluids, gases, or other substances into the zone.

The well system 41 includes an entry well bore 43, well 45, one or more articulated well bores 47, a cavity 49, and a rat hole 51. Entry well bore 43 extends from the surface 53 towards the subterranean zone 55. Well 45 extends from the terminus of entry well bore 43 to the subterranean zone 55, although well 45 may alternatively extend from any other suitable portion of entry well bore 43. Where there are multiple subterranean zones 55 at varying depths, as in the illustrated example, well 45 extends through the subterranean zones 55 closest to the surface into and through the deepest subterranean zone 55. Articulated well bores 47 may extend from well 45 into each subterranean zone 55. Cavity 49 and rat hole 51 are located at the terminus of well 45.

Entry well bore 43 is illustrated as being substantially vertical. However, it should be understood that entry well bore 43 may be formed at any suitable angle relative to the surface 53 to accommodate, for example, surface 53 geometries and attitudes and/or the geometric configuration or attitude of a subterranean resource. Well 45 may be formed to angle away from entry well bore 43. It will be understood that well 45 may be formed at other angles to accommodate surface topologies and other factors similar to those affecting entry well bore 43. In addition, more than one well 45 may be formed at different angular separations from entry well bore 43.

As shown in FIG. 3, the present invention also comprises a method of installing a well, such as a coal bed methane gas well. The method comprises drilling a well bore; installing surface casing 61 in the well bore to a nominal depth; positioning a downhole tool 11 in an initial joint 63 of production casing such that the downhole tool 11 seals against an inner surface of the initial joint 63 of production casing. The downhole tool 11 may be positioned at a lower end of the initial joint 63 of production casing. During installation of the production casing, gas pressure in the well may be handled via an inductor 64 or the like and flared.

The method also comprises installing the initial joint of production casing in the surface casing 61; fishing the downhole tool with a fishing tool 65 to an upper end of the initial joint 63 of production casing adjacent a surface of the well; reading the downhole tool 11 (e.g., pressure gage 31) at the surface of the well for a gas formation pressure of the well beneath the downhole tool 11; installing additional joints 67 of production casing in the surface casing to a desired depth of the well; reading the downhole tool 11 for the gas formation pressure of the well during the previous step; and bleeding off any excessive formation pressure (e.g., in excess of 50 psi) through the downhole tool 11 (e.g., safety relief valve 33) during preceding steps.

While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention. For example, the gas formation pressure at the downhole tool may be read remotely (e.g., by wire, transmitted signal, etc.) to eliminate the need to manually read the pressure gage at the surface of the well. 

1. A downhole tool for installing a well, comprising: a body having a plug shaft including a distal end, a fishing shaft located opposite the distal end, and a bore extending from the distal end; a plug mounted to the plug shaft and having an outer diameter adapted to engage and seal the inner wall of production casing; and a pressure gage in fluid communication with the bore and extending from the fishing shaft, the pressure gage being adapted to measure pressure in the well downwell from the downhole tool.
 2. A downhole tool according to claim 1, wherein the plug shaft has a diameter that is less than a diameter of the fishing shaft, a shoulder is formed at an interface between the plug and fishing shafts, and the plug abuts the shoulder.
 3. A downhole tool according to claim 1, wherein the body has an axis and the bore in the body extends along the axis from the threaded end to the fishing shaft.
 4. A downhole tool according to claim 1, wherein the plug is a cementing plug.
 5. A downhole tool according to claim 1, wherein the distal end of the plug shaft is threaded, and a nut is fastened to the threaded end of the plug shaft for securing the plug to the body such that the plug is axially restrained by the nut.
 6. A downhole tool according to claim 1, further comprising a safety relief valve in fluid communication with the bore and extending from the fishing shaft, the safety relief valve being adapted to relieve excessive pressure in the well downwell from the downhole tool.
 7. A method of installing a well, comprising: (a) positioning a downhole tool in a joint of production casing such that the downhole tool seals against an inner surface of the joint of production casing; (b) installing the joint of production casing in the well; (c) fishing the downhole tool upward in a string of the production casing; (d) reading the downhole tool for a gas formation pressure of the well beneath the downhole tool; (e) installing additional joints of production casing in the surface casing to a desired depth of the well; and (f) reading the downhole tool for the gas formation pressure of the well during step (e).
 8. A method according to claim 7, further comprising bleeding off any excessive formation pressure through the downhole tool.
 9. A method according to claim 8, wherein the excessive formation pressure is a low pressure formation in excess of 50 psi.
 10. A method according to claim 7, wherein the downhole tool is positioned at lower ends of the joints of production casing.
 11. A method of installing a well, comprising: (a) drilling a well bore; (b) installing surface casing in the well bore to a nominal depth; (c) positioning a downhole tool in an initial joint of production casing such that the downhole tool seals against an inner surface of the initial joint of production casing; (d) installing the initial joint of production casing in the surface casing; (e) fishing the downhole tool to an upper end of the initial joint of production casing adjacent a surface of the well; (f) reading the downhole tool at the surface of the well for a gas formation pressure of the well beneath the downhole tool; (g) installing additional joints of production casing in the surface casing to a desired depth of the well; (h) reading the downhole tool for the gas formation pressure of the well during step (g); and (i) bleeding off any excessive formation pressure through the downhole tool during steps (d) through (h).
 12. A method according to claim 11, wherein step (c) comprises positioning the downhole tool at a lower end of the initial joint of production casing.
 13. A method according to claim 11, wherein step (i) comprises low pressure gas formation pressures on the order of 50 psi. 